PARTI-IL PRESSURES O F B I S A R T SOLUTIOSS* BY RALPH Ti-. DORSTE
Introductory The problem of solutions is one which has been studied for many pears although at the present time an adequate theory is still lacking. The partial pressures of the volatile components of a binary solution of liquids h a w frequently been used to study the nature of solutions. I n 1887, Raoult' found empirically a relation involving the partial pressure of a volatile solvent and the concentration of a non-volatile solute. Raoult's original relation is expressed by the equation,
where Siis the number of niols of the solute dissolved in S?niols of solvent, P? the vapor pressure of the pure solvent and P?' its partial pressure in the solution. The mol ratio in the first term refers to the molecular weights in solution. The equation referred t o as Raoult's laJv is
For ideal solutions this relation gives accurately the partial pressures over the entire range of concentrations. In an ideal solution there is no heat effect and no volume change accompanying the mixing of the components. I n this case each coniponent has the same molecular weight in the pure liquid as in the solution and vapor. Raoult's l a x is obviously the expression for the change in the partial pressure of one component as its concentration in the liquid phase is changed by another component whose only effect is dilution. K h e n the partial pressure of the volatile component is plotted against the composition of the liquid phase, Raoult's law gives a straight line joining the zero and the vapor pressure of the pure component. For ideal solutions of two volatile liquids, Raoult's law gives accurately the partial pressure of either component over the entire range of concentrations. The number of ideal solutions of tLvo volatile components is relatively small. For most Folutions it is applicable in the range of the so-called dilute solution.
